I need a long telephoto lens for birds, not less than 500mm to work with a 70D on video and came across an old FD 500mm f4.5 L manual, for about the price of a 100-400. Will the FD to EOS adapter give a significant drawback in quality? Don't need autofocus nor IS for video, just a great pulse. The extra advantaje would be to get a 2X FD..
Note: so far I've been doing wildlife with video cameras but I am switching to EOS because I also do time lapse. I used up to 600mm FD lenses on old betacam camcorders in the past, of course you couldn't even breath beside...
The thing to be aware of is the crop factor, a 500mm FD lens will give the view equivalent to roughly 800mm on the 70D.
I've got a couple of FD lenses that I use with adaptors and I'm very happy with them, not the 500mm f4.5 so I can't comment on that, but I don't think theres any automatic reason to suppose the quality wouldn't be fine.